The present invention relates to a brazing powder composition for use in joining objects, such as mechanical parts, elements, members and the like of aluminum or aluminum alloy, and a method of joining such parts, etc. using such brazing powder composition. One of its possible applications is a manufacture of a heat exchanger assembly.
As means to join two or more objects of aluminum or aluminum alloy by brazing, a brazing material is provided in a powdery form, and is applied to the joint surfaces of the objects being joined. A typical example is disclosed in U.S. Pat. No. 3,971,501 which describes that the brazing powder may be used with no or little limitations to the shape, form or the like of the point of the joint, and may be easily applied even in those areas where some objects, such as complicately configured items, are difficult to be arranged for which any of the prior art brazing materials could be used. In this respect, the above U.S. patent has wide attention.
The brazing powder disclosed in the above patent uses Al--Si eutectic alloy having a melting point below that of aluminum or aluminum alloy parts being joined by brazing. In order to improve a corrosion resistance of the joint, another U.S. Pat. No. 5,547,517 discloses a brazing powder that includes Al--Si eutectic alloy and a small amount of Zn added thereto. A further U.S. Pat. No. 5,251,374 discloses a brazing powder that includes a mixture of Al--Si alloy powder and Zn powder. In U.S. Pat. No. 5,547,517, the brazing powder comprises at most 5% by weight of Zn to be added, while in U.S. Pat. No. 5,251,374, the brazing powder includes at most 3% by weight of Zn to be mixed. An exception to the two patents is Japanese patent application, as now open to a public inspection under Heisei 7-314177, which proposes a brazing powder that is composed of Al--Si--Zn alloy containing 10 to 50% by weight of Zn. Another Japanese application, also now open to a public inspection under Heisei 7-88689, discloses a brazing composition containing not more than 20 parts by weight of Zn.
It is noted that the component of Zn in the brazing composition has a melting point lower than that of Al--Si alloy components, and thus may melt earlier during a brazing process. Accordingly, Zn may tend to move from a coating area toward an area where a fillet is to be formed, and at where Zn may be concentrated. In the portion of the coating area to which Zn has moved, therefore, the satisfactory corrosion resistance cannot be provided by Zn because of its lower content. Alternatively, it may be possible to provide a brazing powder composition that containing more Zn. In this case, however, Zn may have an increased concentration that is more than as required, in the area where the filet is to be formed. When this happens, Zn acts predominantly, which may cause Zn to erode the fillet and others. Thus, it affects the corrosion resistance disadvantageously.
The brazing powder composition that contains Zn as described provides no satisfactory corrosion resistance. For example, in heat exchangers, such as that mounted on auto-vehicles, that must meet the requirements for high corrosion resistance, Zn is previously sprayed thermally, forming a Zn coat onto the surface of the aluminum parts being joined by brazing. Particularly, European patent application, as now open to the public inspection under No.0140267, discloses a process that includes forming a Zn-deposited layer. It is noted, however, that this process involves more steps, increasing the manufacturing cost.
For the joint that includes a large gap between two parts being joined by brazing, such as the joint between tubes and headers in the heat exchanger assembly, or for the joint that requires a large fillet to be formed, the amount of brazing powder used for brazing increases considerably, thus increasing the amount of brazing powder to be coated accordingly. With the increased coating, several coatings are required, as the amount of coating that can be achieved by a single coating (such as by flow coating) is limited. This increases the coating steps, resulting in the higher manufacturing costs. In general, the brazing powder is expensive so that the material cost increases with the increasing usage of the brazing powder.
The inventors of the current application studied the problems associated with the prior art brazing powder, and have discovered that when the brazing powder that includes the hypereutectic Al--Si alloy is used, a base metal is eroded (melted) during the brazing process, and part of the base metal that has been eroded provides the brazing action together with the coating of the brazing powder that has been applied. Then, the inventors have found that the use of brazing powder and therefore the number of its coatings can be reduced while maintaining the good brazeability. It is noted that PCT international publication of No. WO 94/29072 and Japanese patent publication of No. Heisei 8-511201, both of which are open to the public inspection, disclose a brazing material composition that contains more Si, but does not describe that the hypereutectic alloy provides the similar results as described above.
It is also noted that when the Al--Si hypereutectic alloy is used as a brazing material, Si becomes crystallized and crystallized Si causes local corrosion due to the cathode or other corrosion that may form pits. That corrosion may reduce the corrosion resistance in the brazed joints or the areas on which the coating of the brazing powder has been applied. To avoid this problem, more steps must be done so that Zn coating is formed as described.
Because of those problems, the conventional brazing powder is less economical.